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Generating UAV electrical power in flight

Posted: 13 Jul 2012     Print Version  Bookmark and Share

Keywords:unmanned aerial vehicle  exhaust-heat thermoelectric generator  3D modelling 

A one- or two-cylinder, two-stroke engine powers a mid-size unmanned aerial vehicle (UAV). Some of the engine's mechanical output typically is used to drive an alternator to power onboard electronics. A small two-stroke engine converts the energy output of petrol at an efficiency rate less than 20 per cent on average.

As smaller UAVs are designed with more sensors and communications technology for longer missions, the additional electrical power to run them drives the need to generate onboard electric power. One way to create onboard electrical power would be to harness the remaining 80 per cent "waste energy" produced by the two-stroke engine.

A team of engineers at Electronic Cooling Solutions worked with John Langley and engineers at Ambient Micro to build an exhaust-heat thermoelectric generator (EHTEG) that can be incorporated into a UAV design to harvest and convert this waste energy into electrical power in flight [1] (figure 1). The engineers at Electronic Cooling Solutions did the initial EHTEG design, as well as analysing and optimising the thermal design. Then Langley's team built, tested, and redesigned the generator based on the test results.

Figure 1: UAV with the EHTEG attached on top.

Energy in a small engine is wasted as the heat that is lost to cool the cylinder and cylinder head, loss from friction, and the heat of the exhaust stream. Using the heat from the cylinder and cylinder head is too complicated because it would interfere with the process of keeping the entire engine cooled during operation. Energy lost from friction is difficult to access, and it's not a significant part of the overall energy loss anyway.

The best choice is the heat that is lost in the exhaust stream because it usually is about the same amount of power, or more, as the power delivered to the shaft, and it's easy to get to.

The EHTEG had to be mechanically robust and integrate into the aircraft without compromising flight safety. It had to extract the required heat without impairing engine performance. It had to provide the largest possible temperature differential across the thermoelectric modules while operating within the maximum temperature limits of the thermoelectric modules. And it had to be designed with minimal weight and aerodynamic drag.

Designing interior and exterior TEGs
Heat exchangers on the inside of the muffler absorb heat from the exhaust as it flows through. The heat passes through exchangers that line the inside of the UAV's aluminium skin to 2-in. square TEGs mounted on the outside of the UAV and finally passes through another row of heat exchangers to the open air. As the TEGs are exposed to the temperature difference between the hot inside exhaust air and the cool outside air, they generate electric current. The greater the temperature difference, the more current is generated.

The team modelled the thermal design of the system using Mentor Graphics FloTHERM computational fluid dynamics (CFD) 3D modelling software [2]. They simulated airflow on the outside (cool air) and the hot exhaust inside to estimate the temperature difference, which enabled them to optimise the internal and external fins of the heat exchanger and the number and location of the TEGs.

They built engineering models of several likely EHTEG configurations and ran them on a test stand using the same 3W80xi engine and propeller that is used in a MLB Company Bat4 UAV. They validated the FloTHERM CFD models for a range of operating parameters that simulate flight conditions by carefully measuring temperatures, electrical output, and exhaust flow rates using a custom-built airflow test chamber. Exhaust gas composition for calculating mass flow and specific heat was derived from previous work [3, 4].

They started with the internal volume and length for the muffler recommended by the manufacturer that was required to make the system act as an efficient expansion chamber exhaust system for the two-stroke engine. These were used to develop the first half-symmetry CFD models that would determine the number of TEGs needed to optimise the electrical output with minimal weight. The model is symmetrical about a longitudinal vertical plane, so building a half-symmetry model reduced the number of elements down to 1.04 million cells with no loss in accuracy.

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